What is described here is a vehicle brake system for a land vehicle which enables regenerative braking, that is to say, a braking operation in which an electric machine converts kinetic energy of the land vehicle into electrical energy.
As a rule, vehicles have a vehicle-braking installation in which, for braking purposes, a hydraulic fluid is conducted to wheel brakes of a vehicle. Increasingly, however, vehicles also have an electric machine which serves to drive the vehicle in a manner which at least provides assistance to an internal combustion engine. Compared to an internal combustion engine, the electric machine offers the advantage that, when so-called “regenerative braking” occurs, it can be operated in a generating manner in order to charge a vehicle battery. The chemical energy stored in the charged battery is then reused, in a motor mode of the electrical machine, for driving the motor vehicle.
Whereas, in normal braking mode, motional energy of the vehicle is converted into thermal energy because of the frictional interaction of brake shoes and brake discs at the wheel brakes, in regenerative braking mode, charging of the vehicle battery thus takes place. Since, in regenerative braking mode, the braking of the motor vehicle takes place by means of the electric machine, the wheel brakes can remain un-actuated. As a rule, this is even desirable, since any motional energy converted into thermal energy at the wheel brakes is no longer available for charging the vehicle battery.
Thus, for example, document DE 10 2009 039 207 A1 describes a regenerative vehicle brake and a method of operation for a regenerative vehicle brake which has a regenerating apparatus which enables an asymmetric regenerative braking operation for a first and a second hydraulic brake circuit.
However, the vehicle brake described in document DE 10 2009 039 207 A1 comprises more components than conventional brake systems. These additional components require additional installation space and increase the weight of the system. In addition, pedal reaction-simulating apparatuses which are employed in regenerative vehicle brakes often involve more complicated control of the brake system or exhibit, in some cases, an unsatisfactory quality of control.
A large number of measures are known from the prior art for the purpose of changing the braking behaviour in conventional braking operations, that is to say, braking operations without the recovery of energy. Thus, for example, document EP 1 888 387 B1, and corresponding U.S. Pat. No. 8,668,280 B2, both of which are incorporated by reference herein, describes back-pressure control. For this purpose, a pump and a valve of a wheel brake valve arrangement are provided, wherein the valve comprises a throttle which is operative when the valve is in the open state. By actuation of the pump, a back pressure is generated on the input side of the wheel brake when the valve is opened. As a result of the back pressure which is produced, it is possible to compensate, when automatic braking operations occur, for disruptive influences—such as, for example, unequal air clearances between brake pistons and brake linings, unequally worn brake linings or displaced brake pistons. In addition, it is possible to avoid unfavourable characteristics of the pump at a low delivery output.